fix(cookies): 去重服务端重复 cookie 写入

[pjpjq]

What changed

• Wrap server-side cookies.setAll with a duplicate-payload guard.
• Skip a repeated identical cookies+headers batch for the same setAll handler, while still allowing different payloads through.
• Add regression coverage for duplicate and changed payload writes.

Why

When multiple server clients are created during the same SSR request, identical refresh writes can call the same response cookie writer repeatedly and emit duplicate Set-Cookie headers.

Fixes #144.

Validation

• pnpm test src/cookies.spec.ts --run
• pnpm build
• git diff --check